The present invention relates to an image processing apparatus, a program and an image diagnostic apparatus which improve spatial resolution of an image.
Conventionally, in many X-ray CT (Computed Tomography) apparatuses, an arithmetic operation for overlay with a reconstruction function is performed on projection data acquired by imaging, and back projection processing is executed thereon to thereby reconstruct an X-ray CT image.
The quality of the X-ray CT image depends on the characteristic of the reconstruction function used in image reconstruction. Therefore, in each X-ray CT apparatus, a plurality of types of reconstruction functions respectively different in the quality of the reconstructed image are prepared and provided to a user. There are prepared, for example, a lung field function close to high spatial resolution so adjusted that a high-frequency component appears relatively strongly, a soft part function close to low noise so adjusted that a high-frequency component appears relatively weakly, a standard function having an intermediate property between these, etc. The user properly uses these reconstruction functions according to diagnostic purposes, sections to be observed and the like. See, for example, paragraphs [0021], [0029] and [0030] of Japanese Patent Application Laid-Open No. 2004-073432.
On the other hand, when an image is reconstructed using a reconstruction function, spatial resolution and noise level in the reconstructed image are in a trade off relationship with respect to each other. Therefore, when such a reconstruction function that the high-frequency component appears extremely strongly is used in an attempt to enhance the spatial resolution to the maximum, an increase in noise becomes sharp so that the image may result in an image that does not withstand a practical use.
With this situation, in regard to the previously-prepared reconstruction function, the balance between spatial resolution and a noise level has been adjusted within a range durable for practical use. Therefore, even in the case of the reconstruction function close to the highest spatial resolution, potential spatial resolution of projection data has not yet been drawn to a maximal degree.
On the other hand, a section (e.g., auditory ossicles or the like) having a very fine structure even within the sections of a subject has been desired at a higher spatial resolution. There has been room for further high spatial resolution. However, when using the reconstruction function adjusted such that an improvement in spatial resolution is pursued at random, and such that the high-frequency component appears strongly, noise is increased needlessly with respect to a region that does not require such a high spatial resolution (e.g., a soft tissue region), thus leading to an undesirable result.
With the foregoing in view, a process capable of making a further improvement in spatial resolution for the region without increasing noise needlessly is desired.